1. Field of the Invention
The present invention relates to a solid-state image pickup apparatus with an array of a plurality of pixels each having a photoelectric conversion function, and an image pickup system using the solid-state image pickup apparatus.
2. Related Background Art
Solid-state image pickup elements are roughly classified into charge transfer type elements such as a CCD, and XY address type elements such as a MOS image pickup device.
Using this solid-state image pickup element as a sensor provides not only many merits but also demerits. One of the demerits is image deterioration called as “smear” generated when a bright object image is picked up.
In the CCD, part of object light leaks as photo-leak to a vertical transfer register adjacent to a photodiode, and is observed as a white noise band on the image in the vertical direction of the bright object image. This phenomenon occurs with a light quantity corresponding to about 60 dB to about 100 dB of light amount with which the photodiode saturates.
In the MOS image pickup device, as shown in FIG. 1, the pixel is generally made up of a photodiode 1, an amplification MOS transistor 2 for amplifying and outputting a signal from the photodiode, a transfer MOS transistor 3 for transferring the signal from the photodiode 1 to the amplification MOS transistor 2, a reset MOS transistor for supplying a reset potential to the gate region of the amplification MOS transistor, and a selection MOS transistor 5 for selectively outputting a signal from the amplification MOS transistor 2. It has been understood that the smear can be avoided substantially when the gate electrode region of the amplification MOS transistor is reset before the signal from the photodiode 1 is transferred to the amplification MOS transistor 2. For example, the transfer time from the photodiode to the amplification MOS transistor is several μs, while an exposure time is 16 or 17 ms for a movie camera such as a video camera. The light quantity difference is about 100 dB, and the gate electrode region of the amplification MOS transistor is shielded from light. In this situation, it has been understood that smear is considered not to occur.
However, the present inventors have conducted image pickup experiments to find the following problems.
There is a conventional solid-state image pickup apparatus having a difference means for subtracting a noise component in order to remove a noise component contained in a signal photoelectrically converted and output by a photoelectric conversion means such as a photodiode.
An example of the solid-state image pickup apparatus having the difference means is shown in FIG. 2. FIG. 3 is a timing chart showing the operation timing of the solid-state image pickup apparatus in FIG. 2.
A pixel 6 has the same structure as that of the pixel shown in FIG. 1. A reset signal vn obtained when the input of an amplification MOS transistor is reset, is stored in a memory CN7 during a period t1, and a signal vs generated by photoelectric conversion by a photoelectric conversion means is transferred to the input of the amplification MOS transistor during a period t2. At the same time, a signal VS output from the amplification MOS transistor is stored in a memory CS 8. The signal VS stored in the memory CS 8 contains the signal (vs) generated by photoelectric conversion and the reset signal (vn).
The reset signal vn stored in the memory CN 7 and the signal VS (=vs+vn) stored in the memory CS 8 are read out to a differential amplifier 9. The differential amplifier calculates difference VS−vn, and outputs a signal vs free from any noise component, from which signal vs the reset signal as a noise component is removed.
The input of the amplification MOS transistor should be reset during the period t1, but a photo-leak noise signal v1 is added to the reset signal which is a noise component, owing to photo-leak of very strong light (VN=vn+v1). Hence, an output signal from the differential amplifier is VS−VN=vs−v1. If v1 saturates, the output signal vs−v1 becomes 0, and the image becomes darkened regardless of a bright object.
FIG. 4 is a conceptual view showing this phenomenon. The abscissa represents the incident light quantity on the photoelectric conversion means, and the ordinate represents the level of a signal generated in the photoelectric conversion means.
When, for example, a bright object (the sun, a light source, or the like) exists in an object to be picked up, the corresponding portion becomes a darkened image depending on image pickup conditions, resulting in low image quality.